Principles of Operation

The vehicle has two module communication networks. The Standard Corporate Protocol (SCP) communication network, which is an unshielded twisted pair cable (data bus plus, Circuit 914 [TN/OG] and data bus minus, Circuit 915 [PK/LB]) and the International Standards Organization (ISO) 9141 communications network which is a single-wire network(Circuit 70 [LB/W]). Both networks can be connected to New Generation STAR (NGS) Tester by one connector called the Data Link Connector (DLC). This makes diagnosis and testing these systems easier by allowing one smart tester to be able to diagnose and control any module on the two networks from one connector. The DLC can be found under the instrument panel between the steering column and the radio.

The ISO 9141 communication network does not permit intermodule communication. When NGS Tester communicates to modules on the ISO 9141 communication network, NGS Tester must ask for all information; the modules cannot initiate communications.

The SCP communication network will remain operational even with the severing of one of the bus wires. Communications will also continue if one of the bus wires is shorted to ground or battery positive voltage (B+), or if some but not all termination resistors are lost.

Unlike the SCP communication network, the ISO 9141 communication network will not function if the wire is shorted to ground or battery positive voltage (B+). Also, if one of the modules on the ISO 9141 communication network loses power or shorts internally, communications to that module will fail.

The Electronic Crash Sensor (ECS) is connected to the ISO 9141 communication network. The ECS controls the deployment of the air bags based on sensor input.

The Driver Door Module (DDM) is connected to the SCP communication network. The module controls many functions including power windows, power locks and keyless remote entry.

The Lighting Control Module (LCM) is connected to the SCP communication network. The LCM controls both interior and exterior lighting, anti-theft functions and warning chimes.

The Electronic Automatic Temperature Control (EATC) module is connected to the SCP communication network. The EATC module controls automatic climate functions that maintain the vehicle at a constant temperature setting.

The Powertrain Control Module (PCM) is connected to the SCP communication network. The PCM controls the engine performance, electronic ignition, emission controls and on-board diagnostics.

The Natural Gas Vehicle Module (NGVM) calculates fuel level based on inputs from the fuel tank pressure sensor and the fuel tank temperature sensor. The NGVM then provides a voltage signal to the instrument cluster to drive the fuel gauge. The NGVM also has separate injector drivers to run the injectors at the higher operating pressure of natural gas. The NGVM does not perform any calculations for this function; calibration for the injectors is maintained in the Powertrain Control Module (PCM).

The anti-lock brake control module is connected to the SCP communication network. The module controls the brake pressure to the four wheels to keep the vehicle under control while braking.

The Passive Anti-Theft System (PATS) module is on the SCP communication network. The PATS module communicates with the PCM to disable and start the vehicle based on a signal received from an encoded ignition key signal.